Various apparatuses for processing materials with microwave or radio frequency (RF) energy in closed chambers have been developed for home, commercial and industrial applications. The most well-known example is the ubiquitous microwave oven where, typically, a single source of microwave energy, a magnetron, delivers microwave energy to a rectilinear chamber through a waveguide or waveguide horn antenna with fixed polarization (polarization is a parameter that identifies the orientation of the electric field component the electromagnetic field in space and time). The operating frequency is usually selected as one of the standard industrial frequencies. The selected standard frequency is a result of a compromise between the absorption skin depth in the load material, efficiency of the source (usually a magnetron), and dimensions of both the load and the source including its power supply.
The deficiency in this basic approach is that the distribution of microwave energy is generally very non-uniform and inefficient. The microwave energy density is non-uniform because the resonant modes of the chamber, determined by the frequency of the magnetron and the dimensions of the chamber having typically a single power coupler, create wave patterns that can add both constructively and destructively (the resonant modes are known as Eigenmodes, which are solutions to the electromagnetic wave equations under the boundary conditions imposed by the chamber and the coupler and antenna). As a result, the distribution of microwave energy in the chamber is very non-uniform and the microwave oven generally exhibits hot spots and cold spots in a load. To remedy this deficiency, microwave oven manufacturers have introduced “stirring” mechanisms, which are essentially metallic “propellers” that constantly change the boundary conditions of the chamber to redistribute the microwave energy in the chamber. Another common approach is to provide a rotating food platform that moves the food in and out of the hot and cold spots in an attempt to average out the non-uniformities over the cooking time. The microwave ovens are inefficient because the impedance of the loaded chamber (dominated usually by the water content of the load, its distribution and the volume to be heated) as measured, for example, at the coupler port, is highly variable unlike the impedance of the microwave power source (a basic principal of power transfer efficiency is a match between the impedance of the source and the impedance of the loaded chamber). However, these approaches add cost and complexity, reduce reliability, limit minimum processing time, and are not generally applicable to higher power industrial applications such as heating, drying, sterilization, disinfection, polymerization, and chemical synthesis.
Conventional industrial chambers suffer from the same limitations as microwave ovens, and other limitations as well. Compared to home or commercial microwave ovens, industrial chambers used for heating, drying and chemical synthesis must often operate at much higher power levels (10's of kilowatts versus 1-2 kilowatts). Typically, these chambers are fed by two or more open-ended waveguides or horn antennas that can handle the high power levels, and which are rigidly fixed to the chamber wall. Variations in the load (the material that is being irradiated by the microwave energy), in terms of volume, density, distribution and dielectric constant, for example, can disrupt the distribution of resonant modes in the chamber, resulting in poor uniformity and efficiency. Having more than one coupler in a processing chamber helps to improve uniformity of processing, but also creates problem of mutual influence of these couplers (sources) known as intercoupling or cross-coupling. Additionally, it is very difficult to control cross-coupling between the antennas, which can detune the microwave sources and lead to further losses in uniformity and efficiency.
One approach to overcome these limitations is to employ a single-mode chamber, typically of dimensions smaller than approximately one wavelength, to support only one mode within the operating band of the sources. As a result, the maximum load size in single-mode chambers is less than a cubic wavelength or, for example, about 1 liter at 2.45 GHz. In order to process larger loads, chambers with dimensions larger than approximately one wavelength are required, but existing approaches do not adequately address the limitations of source intercoupling and interference mentioned above.
Other conventional approaches rely on “cross-polarization” between electromagnetic fields radiating from two different sources, which is the condition where the polarization plane, usually defined by the electric field component and direction of radiation propagation, emitted by one radiating element is perpendicular to that emitted by a second radiating element at all points within the volume of interest. Cross-polarized fields do not interfere, even if the corresponding sources are completely synchronized or coherent, such as when two radiating elements are driven by the same source, and so the time average power does not exhibit spatial or temporal interference fringes.
As is known in the art related to closed structures, cross-polarization is usually accomplished in rectangular waveguides or parallelepiped chambers so that the excited mode polarizations are perpendicular at every point (see, e.g., FIGS. 1-2 in U.S. Pat. No. 4,795,871). The '871 patent specifies conical and pyramidal walls that are not parallel or perpendicular but the orientation of the radiators is implied in FIGS. 3-8 as either parallel or perpendicular to the plane containing polar axis and the central point of the radiator.
The analysis in the '871 patent is based on essentially traveling waves propagating as an optical beam in an open space. In the presence of a non-rectilinear, closed chamber of dimensions comparable to approximately ten wavelengths, commonly used in domestic and industrial applications, the fields exist in a form of a discrete set of standing waves exhibiting a pattern of maxima and minima determined by the chamber geometry and its contents. The polarization of these standing waves in general are not mutually perpendicular at all points and therefore it is not obvious that any arrangement of multiple radiating elements can excite non-intercoupled modes.